Tick-borne encephalitis causes
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Ilan Dock, B.S.
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Overview
Life Cycle and Spread of Disease
General Tick Life Cycle [1]
- A tick's life cycle is composed of four stages: hatching (egg), nymph (six legged), nymph (eight legged), and an adult.
- Ticks require blood meal to survive through their life cycle.
- Hosts for tick blood meals include mammals, birds, reptiles, and amphibians. Ticks will most likely transfer between different hosts during the different stages of their life cycle.
- Humans are most often targeted during the nymph and adult stages of the life cycle.
- Life cycle is also dependent on seasonal variation.
- Ticks will go from eggs to larva during the summer months, infecting bird or rodent host during the larval stage.
- Larva will infect the host from the summer until the following spring, at which point they will progress into the nymph stage.
- During the nymph stage, a tick will most likely seek a mammal host (including humans).
- A nymph will remain with the selected host until the following fall at which point it will progress into an adult.
- As an adult, a tick will feed on a mammalian host. However unlike previous stages, ticks will prefer larger mammals over rodents.
- The average tick life cycle requires three years for completion.
- Different species will undergo certain variations within their individual life cycles.
Spread of Tick-borne Disease
- Ticks require blood meals in order to progress through their life cycles.
- The average tick requires 10 minutes to 2 hours when preparing a blood meal.
- Once feeding, releases anesthetic properties into its host, via its saliva.
- A feeding tube enters the host followed by an adhesive-like substance, attaching the tick to the host during the blood meal.
- A tick will feed for several days, feeding on the host blood and ingesting the host's pathogens.
- Once feeding is completed, the tick will seek a new host and transfer any pathogens during the next feeding process. [1]
Transmission
- The Ixodidae family of hard ticks have been reported as the vector and reservoir of the Tick-borne encephalitis virus.
- Other modes of transmission include the consumption of raw milk as well as vertical transmission from mother to fetus.
Virology
- Member of the Falvivirus genus
- Flaviviridae family
- Three subtypes: Far East, European, and Siberian
- Viral strains are mostly homogeneous within infected European tick populations.
- Diversity exists within viral strains carried by Siberian and Far Eastern tick populations. Thus these populations host antigenic variations and a variety of subtypes.
- However the antigenic similarity within these populations allows for a generalized protection method among the different subtypes.
Genomics
- (+)ssRNA genome enclosed in a capsid protein.
- Genome is protected by a lipid bilayer, provided by the host or target cell.
- Virus's physical attributes include a spherical particle with an approximate diameter of 50-60nm.
- The genome lacks a 3'-poly(A) tail, yet provides a 5' cap.
- In terms of length, the genome spans an average of 11kb.
Pathogenesis
- The process begins as the virus binds to a host cell receptor.
- A host cell will internalize the virus using endocytosis.
- Post-endocytosis, acidification of the viral envelope causes conformation changes of the E protein, resulting in the attachment of the viral envelope to a endosomal vesicle.
- Once properly mounted on the endosomal vesicle, the viral envelope will release the viral nucleocapsid into the surrounding cytoplasm.
- Translation of the virus yields a 3414 amino acid long polyprotein.
- The polyprotein is cleaved by both cellular and viral proteases.
- The cleaving process results in three structural proteins called C, prM, and E as well as seven non-structural proteins.
- The C protein forms a virion nucleocapsid through binding to viral DNA.
- The E protein is necessary as a ligand to cell receptors and as a fusion protein.
- The other non-structural proteins serve as proteases, polymerases, complement binding antigens, or function within the replication process.
- Finally the processes concludes as the positive-stranded genome is translated while the negative-strand of RNA provides grounds for the RNA replication process.
- Assembly of the virus occurs within the endoplasmic reticulum.
- Post-assembly immature virions are released within the cell.
- ↑ 1.0 1.1 Life Cycle of Ticks that Bite Humans (2015). http://www.cdc.gov/ticks/life_cycle_and_hosts.html Accessed on December 30, 2015